Method of drying leather



- Patented Aug. 16, 1938 PATENT OFFICE METHOD or name LEATHER. James R. Akcrs, New Kensington, Pa., assignor umin toAl

um Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

7 Claims.

This invention relates to improvements in the drying of leather, and particularly to the drying of leather after tanning, on oxide-coated aluminum drying boards. j

In the tanning process, to produce leather from hides, the leather as it comes from the tanning baths is thoroughly saturated with an acid tanhing liquor. The drying of this leather is generally accomplished by first mechanically pressiO ing the leather to remove a large proportion of the acid solution contained in it, and then mounting the leather on a large flat surface by means of an adhesive and subjecting it to a drying operation, such as in a drying oven, at elevated iii temperature.

Various types of fiat surfaces have been used for stretching the leather in this drying operation, such as composite board, lacquered paper board, and plate glass. More recently it has been proposed to use aluminum or aluminum alloy sheets provided on their surfaces with oxide coatings as drying boards. The aluminum sheets, because of their relatively light weight, are easy to handle, and the protective oxide coating on the aluminum surface gives to these aluminum leather-drying boards a substantially inert surface which has a relatively long useful life. It has been found, however, that on long continued use of these oxide-coated aluminum drying boards the oxide coating is gradually dissolved from the aluminum surface, leaving a bare metallic surface to contact the stretched leather. This bare aluminum surface, on coming in contact with the leather, becomes discolored by the 5 tanning liquors contained in the leather, and

this discoloration will sometimes produce a stain on the leather surface. Accordingly, this removal of the oxide coating from the aluminum leather-drying board surface gradually renders such boards unfit for further use.

It is an object of this invention to increase the useful life of oxide-coated aluminum leatherdrying boards used for the drying of leather after tanning. It is more particularly an object of this invention to provide a method for preventing the deterioration of the oxide-coated aluminum leather-drying boards resulting from the removal of the oxide coating from the surface, and to provide an adhesive material which will act to preserve the oxide-coated aluminum surface.

I have found that the acid tanning liquors contained in the leather to be dried attack the oxide coating, particularly at the. elevated drying temperatures generally used, and that this attack causes theremoval of the oxide coating from alu- Application November 24, 1936, Serial No. 112,525

minum leather-drying boards during use. The acid tanningliquors contained in the leather generally have a pH of 3.0 to 3.5, and the drying temperature is generally upward of about C. Under these conditions, the acid of the tanning 5 liquors is sufiicient to cause slow solution ofthe aluminum oxide of the coating on the aluminum leather-drying boards.

According to my invention, the attack of the acid tanning liquors on the oxide-coated surface can be substantially prevented or reduced by maintaining a pH of at least about 4.5, and. preferably of about 5.5 to 6.0, at the surface of the oxide coating. In carrying out my invention I have found that by-interposing between the wet leather surface and the oxide-coated. aluminum surface on which it is mounted a substance capable of reacting with the acid of the tanning liquor contained in the wet leather to reduce its acidity, the pH at the surface of the oxide coating may be controlled within suitable limits to prevent attack of the particular tanning liquor in question on the..oxide coating at the drying temperature being used. I have found it preferable and most convenient to interpose the neutralizing compound between the oxide coating and the wet leather by incorporating it in the adhesive used in mounting the-wet leather on the oxide-coated aluminum surface. I

The mounting of the wet leather on the oxide; coated aluminum leather-drying boards is generally accomplished by adhering the leather to the oxide-coated surface by means of one or more adhesives of the paste type. In some cases, at least two adhesives are used, one of a lighter type being used in thecenter of the leather piece and a heavier paste being used around the outer edge. I have found that by incorporating in the adhesives used to mount the leather on. the drying board a normally inert substance capable of reacting with the tanning liquor from the leather to produce at the surface of the oxide coating 9. pH of at least about 4.5, deterioration of these oxide-coated aluminum leather-drying boards can a be substantially prevented.

The compound incorporated in the adhesive according to my invention should be one which does not adversely affect either the oxide coating or the leather itself and one which does not destroy the bonding properties of the adhesive. 50 It is preferable to incorporate in the adhesive a substantially insoluble compound which will react with the acid of the tanning liquor to form an insoluble compound and reduce the acidity of the liquor. For this purpose I have found the. as

carbonates of the alkaline earth metals to be particularly suitable, although other compounds 'may be used, such as oxides and silicates.

I prefer to incorporate in the adhesive calcium carbonate or materials containing calcium carbonate, such as chalk, limestone and the like. The concentration of the alkaline earth compound in the paste is not critical but must be 'maintained sufiiciently low so that it does not destroy the bonding properties of the paste by its diluting action. In general, at least about 0.05% of the alkaline earth carbonate should be present in the paste, and I prefer, when using calcium carbonate or a material containing calcium carbonate, to use it in amounts of at least 0.1 to 1.0%.

The'method of my invention may be practiced with substantially any of the known adhesives generally used for mounting leather on drying board surfaces. For example, a paste of the type made by boiling together wheat flour and water in the proportion of about eight pounds of wheat flour to twenty-five gallons of water, or a paste made by adding A pound of corn starch, pound of sago flour and A pound of flour to a gallon of water at F., to which has been added about 1% of French chalk containing about 10% calcium carbonate. gives satisfactory protection.

I claim:

l. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising interposing between the leather to be dried and the oxide-coated surface upon which it is mounted a substance capable of reacting with the acid of the tanning liquorfrom the leather, and maintaining a pH at the surface of the oxide coating of at least 4.5.

2. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising incorporating in the adhesive used to mount the leather to be dried on the oxide-coated surface a substance capable of reacting with the acid of the tanning liquor in the leather to increase the pH of said liquor to at least 4.5.

3. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising mounting the leather to be dried on the oxide-coated surface with an adhesive containing a normally inert substance capable of reacting with the acid of the tanning liquor in the leather to increase the pH of said liquor to at least 4.5.

4. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising incorporating in the adhesive used to mount the leather to be dried on the oxide-coated surface a substantially water-insoluble alkaline earth metal compound.

' 5. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising incorporating in the adhesive used to mount the leather to be dried on the oxide-coated surface an alkaline earth carbonate.

6. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising incorporating calcium carbonate in the adhesive used t'o-mount the leather to be dried on the oxide-coated surface. 1

'7. A method of preventing deterioration of oxide-coated aluminum leather-drying boards, comprising interposing between the leather to be dried and the oxide-coated surface upon which it is mounted a substancecapable of reacting with the acid of the tanning liquor from the leather, and maintaining a pH at the surface of the oxide coating of 5.5 to 6.0.

JAMES R. AKERS. 

